The present invention relates to optical bodies and methods of making optical bodies. More specifically, the invention is directed to optical bodies that resist warping when cycled through temperature changes, and to methods of making such optical bodies.
Multilayer polymeric optical films are widely used for various purposes, including as mirrors and polarizers. These films often have extremely high reflectivity, while being lightweight and resistant to breakage. Thus, the films are well suited for use as reflectors and polarizers in compact electronic displays, including as liquid crystal displays (LCDs) placed in mobile telephones, personal data assistants, and portable computers.
Although polymeric optical films can have favorable optical and physical properties, one limitation with some such films is that they show significant dimensional instability when exposed to fluctuations in temperaturexe2x80x94even the temperature fluctuations experienced in normal use. This dimensional instability can result in formation of wrinkles in the film as it expands and contracts. Such dimensional instability is particularly common when temperatures approach or exceed approximately 80xc2x0 C. At these temperatures the films fail to maintain a smooth, flat surface, and form wrinkles as a result of warping. This warping is often particularly pronounced in larger films, such as those used in desktop LCD monitors and notebook computers. Warping is also observed when the film is cycled to high temperature and high humidity conditions, such as conditions of 60xc2x0 C. and 70 percent relative humidity.
Thus, a need exists for improved optical films that have favorable optical properties, but which limit warping caused by dimensional instability.
The present invention is directed to multilayer optical bodies that resist warping when exposed to temperature fluctuations. In particular, the invention is directed to multilayer optical bodies that contain an optical film joined to one or more dimensionally stable layers. The dimensionally stable layers support the optical film such that the composite multilayer optical body resists warping after exposure to temperature fluctuations, while maintaining the light weight, durability, and flexibility of the optical film. The invention is also directed to methods of making optical bodies and to displays (such as LCDs) containing the optical bodies.
In addition to containing an optical film and one or more dimensionally stable layers, the optical body can include one or more additional layers. These additional layers can be placed between the optical film and the dimensionally stable layer, and can include an extrudable thermoplastic polymer that secures the optical film and dimensionally stable layers together.
The optical film used in the mutlilayer optical body includes, for example, polarizing reflectors and mirror films. The optical film is typically an oriented optical film, and is itself typically composed of multiple layers. In certain embodiments the optical film includes at least one layer containing polyester, such as polymers or copolymers of poly(ethylene naphthalate) (PEN and COPEN) and polymers or copolymers of polyethylene terepthalate (PET and CoPET).
The dimensionally stable layer adds sufficient dimensional stability to the composite multilayer body such that it does not substantially warp when exposed to changes in temperature. The dimensionally stable layer is typically a polymeric material exhibiting a glass transition temperature (Tg) from 80 to 250xc2x0 C., more typically from 100 to 200xc2x0 C. Suitable dimensionally stable layers can include polycarbonate or miscible blends of polycarbonate and PEN, COPEN, PET, and CoPET. Additional suitable dimensionally stable layers contain polystyrene or polystyrene copolymers, including styrene acrylonitrile, acrylonitrile butadiene styrene, and styrene methyl methacrylate. High Tg nylons and polyetherimide may also be used for to add dimensional stability.
The optional additional layer or layers can serve various functions, and typically serve to promote adhesion of the optical film to the dimensionally stable layer. The additional layers can be an intermediate layer, are typically a polymeric composition, and are preferably thermally stable in the melt phase at temperatures above 250xc2x0 C. Thus, such intermediate layers do not typically show substantial degradation during extrusion at temperatures greater than 250xc2x0 C. In specific implementations the intermediate layer is transparent or substantially transparent. In some implementations, the intermediate layer can contain particles of differing refractive index to improve diffusion.
The optical bodies of the present invention can be formed by methods in which the dimensionally stable layer is co-extruded or extrusion coated onto the optical film. When the optical film is an oriented optical film, it can be oriented by stretching either before or after being joined to the dimensionally stable layer. Additional layers, when present, are also typically extrusion-coated onto the optical film. The individual layers should have sufficient thermal stability to undergo co-extrusion or extrusion coating at elevated temperatures.